1. Field of the Invention
The present invention relates to a rolling bearing and a rolling bearing device, each of which is to be used in a spindle motor for driving a hard disk, a polygon mirror or the like.
2. Description of Related Arts
There is known a spindle motor for rotationally driving a magnetic disk serving as a recording medium. A magnetic disk is generally supported by a rotor hub supported rotatably relatively to a stationary shaft member. The spindle motor of this type includes a ball bearing made of a steel-type material for supporting the inner periphery of the rotor hub directly or indirectly through a spacer made of a steel-type material.
Magnetic disks made of aluminium disks are generally used, but magnetic disks mainly composed of glass disks are now also widespread.
When a rotor hub made of aluminium is used for a magnetic disk of aluminium, the rotor hub will not be greatly deformed in shape due to temperature variations because the rotor hub and the magnetic disk are the same in coefficient of thermal expansion.
However, when a rotor hub made of aluminium is used for a magnetic disk of glass, the rotor hub will be relatively greatly deformed in shape. If the rotor hub is relatively greatly deformed in shape, the magnetic disk supported by the rotor hub is moved from the regular position or changed in surface angle with respect to a reading/writing head from the regular surface angle. This involves the likelihood that the reading device makes a reading error and/or a writing error with respect to a magnetic disk, and that the magnetic disk or the reading/writing head is damaged.
For a glass-made magnetic disk, it is therefore preferable to use a rotor hub made of stainless steel lower in coefficient of linear expansion than aluminium.
On the other hand, to improve the lubrication lifetime under high-speed and high-temperature conditions, a hard disk driving spindle motor is increasingly using a rolling bearing comprising balls made of silicon nitride.
To obtain high rotational precision, the spindle motor above-mentioned is used with a pre-load applied to the ball bearing. In the spindle motor of this type required to be miniaturized, there is applied a so-called fixed-positional pre-load without the use of a spring or the like. However, the coefficient of linear expansion of silicon nitride is about 1/3 of that of bearing steel generally used for inner/outer rings. Thus, the pre-load disadvantageously undergoes a great change due to temperature variations.
For example, when at low temperature, the pre-load increases and exceeds the allowable limit, the bearing torque is increased to increase the power consumption of the motor. This is contradictory to energy saving. On the other hand, when at high temperature, the pre-load excessively decreases, the rotational precision is lowered to produce reading and/or writing errors.